While I know pearls come in many shapes, and I can see why MarcA has made the suggestion, I wonder if it would be easy to find a matching set in the shapes seen in the photographed necklace.

The Princess was recorded as having worn a turquoise tiara and necklace for a Court in May 1903, just shortly after the photo was published, so I suspect she was wearing turquoises in the photo.

I suspect, but cannot prove, that this is the same tiara and necklace which Princess Christian regularly wore to Courts and other official occasions.

If the necklace worn by Princess Helena Victoria is the same as that given to QEQM in 1923 it wouldn't be the first piece of jewellery which may have somehow been obtained by Queen Mary from the collection of Princess Helena, Princess Christian. The board discussed 2 other possibilities in 2016. That discussion starts at the heading "Some other jewels unknown to me" http://members2.boardhost.com/royal-jewels/thread/1464944159.html